We have developed a means of comparing cell surface proteins of human tumor with normal tissue from the same individual, utilizing relatively small amounts of tissue obtained at the time of surgery. Lactoperoxidase, unable to enter viable cells because of its high molecular weight, was used to catalyze the addition of iodine to tyrosine in cell surface proteins. To insure that only surface proteins were labelled it was first necessary to obtain a homogeneous population of viable surface-radiolabelled cells. This was accomplished by the use of an albumin density gradient. Resultant viable surface I125 radiolabelled cells were then lysed, and radiolabelled cell surface proteins were sieved on Sephadex G 200. Elution patterns for radioiodinated surface proteins of adenocarcinoma of the stomach and colon and corresponding normal stomach or colon cells from the same person were all similar, i.e. show 3 peaks. All adenocarcinoma, however, contained relatively more radioactivity in peak II than normals (p equals .015). CEA was found to be located in peak I. Thus we have demonstrated that, in addition to CEA, there are other consistent differences between cell surface protein(s) of colon and stomach adenocarcinoma and corresponding normal tissue. Additional work will be required to further characterize these protein(s) and to determine if carcinoma patients have circulating antibodies directed toward them.